As shown in FIG. 5, a conventional hydraulic tensioner 500 comprises a plunger 510 having a hollow portion 511 with a closed end 511b, and open toward the inside of the tensioner. The plunger 510 fits into a cylindrical hole 520 in a tensioner housing 520, and is extensible and retractable in the tensioner housing so that it can maintain engagement with a timing chain. An oil passage 522 allows oil, supplied under pressure from an oil source, to communicate with the cylindrical hole 521. A check valve block 530, which is slidably fitted into the cylindrical inner surface 511a of hollow portion 511, allows oil to flow toward the closed end 511b of the plunger 510. A spring 540, interposed between the check valve block 530 and the closed end 511b, biases the plunger forward, and at the same time holds the check valve block against a sleeve 550, which is seated on an end portion 523a of a hole 523 in the plunger. Hole 523 has a diameter d2 which is smaller than the diameter d1 of the cylindrical hole 521. The check valve block 530 is thus held at a fixed position in the tensioner housing 520 although it is slidable relative to the inner circumferential surface 511a of the plunger 510.
In the conventional hydraulic tensioner 500, the oil passage 522 communicates with an internal space IA of the inner sleeve 550, which functions as an oil reservoir. The oil fills a high pressure chamber HA between the closed end 511b of the hollow portion 511, and the check valve block 530, from the inside of the inner sleeve 550, through the check valve block 530. The plunger 510 is always pressed toward the timing chain by the biasing force of the spring 540, to apply chain tension to the chain.
When the plunger 510 is pushed back into the tensioner housing by an impact applied to the plunger through the timing chain, oil in the high pressure chamber HA is discharged through an oil discharge passage 526, which connects the cylindrical hole 521, to the outside of the tensioner housing 520. The oil flows through gaps between the outer circumferential surfaces of the check valve block 530 and inner sleeve 550, and the inner circumferential surface 511a of the plunger, thereby absorbing, and reducing the effect of, the impact force.
The conventional hydraulic tensioner 500 is disclosed at page 1, and shown in FIG. 1, of Japanese Patent No. 3054068. The conventional tensioner must be subjected to secondary machining to form the oil discharge passage 526, and fine secondary machining to form an oil supply opening 551, which provides for fluid communication between the oil passage 522 and the interior of the inner sleeve 550. The requirement for secondary machining resulted in a significant increase in the time and cost for manufacturing the conventional tensioner. Another problem encountered in the operation of the tensioner was that oil tended to adhere to and clog the oil discharge passage 526 and the oil supply opening 551, especially after the tensioner was in use for a long period of time. The clogging of these passages can result in unstable operating conditions, that cannot be easily remedied.
Another problem with the conventional tensioner was that a significant amount of oil was consumed as a result of discharge through passage 526 when impact forces were encountered. Thus, the engine oil pump was required to have sufficient power to supply oil to the tensioner under varying conditions. Moreover, when the engine is stopped, the engine oil pump also stops, but the discharge of oil from the tensioner cannot be avoided. Under such conditions, the amount of oil in the tensioner body 520 tends to be inadequate when the engine is restarted, and the tensioner cannot properly accommodate impact forces applied through the timing chain until the interior of the tensioner is re-charged with oil. Accordingly, wobbling of the timing chain occurs as the engine is started, producing noise known as a “wobbling sound.”
The objects of the invention are to solve the problems of the conventional hydraulic tensioner; to provide a hydraulic tensioner which is simpler in construction, and can be manufactured in a shorter time and at a lower cost; to suppress the consumption of oil by the tensioner; to achieve stable application of tension; and to eliminate wobbling sounds in the timing transmission medium upon starting of an engine.